I think the answer depends on what you plan to do with your certificate. If you're planning to be a career teacher, then an onsite course is definitely the way to go; however, if you just want to go abroad to teach for 6 months to one year and don't plan to make a career out of teaching, then maybe it's not worth the investment for a 4-week course and an online course is a better option. Many employers will give you additional training to supplement what you learned in an online course.
You'll definitely be better prepared with an onsite course that has practice teaching but not everyone can take 4 weeks out of their lives and if you just want to teach short term in a country that accepts them, online courses are a good option.
